The present invention relates to an internal combustion engine with at least one cylinder, in which a fuel/air mixture can be compressed in the cylinder by a piston.
The constant increase in the use of such internal combustion engines in motor vehicles and for the generation of electric energy has, because of the limited quantities of fossil fuels which form the basis for the preparation of engine fuels, and because of new government legislation limiting the emission of harmful exhaust gases, driven forward the search for new highly efficient combustion concepts, low in pollutants, for such engines.
The HCCI concept (homogeneous compressed charge ignition) may be named here by way of example, in which the ignition of an extremely lean and homogeneous fuel/air mixture is brought about by increasing the temperature during the compression process close to the top dead centre of the piston. With the HCCI concept, the features of conventional combustion concepts are combined in a very advantageous way, since for example, as with Otto engine, a homogeneous fuel/air mixture is used which minimizes particle formation (soot) during the combustion process and, as with the diesel engine, a spontaneous ignition of the fuel/air mixture takes place, whereby a high thermodynamic efficiency results. The use of a lean fuel/air mixture permits its combustion with extremely low nitrogen oxide (NOx) levels.
The spontaneous ignition of the fuel/air mixture in the combustion chamber is mostly achieved by a combination of different measures, such as for example a high geometric compression ratio ε and the pre-heating of the cylinder charge through suitable measures (air preheating or recycling of exhaust gas). Since, with this combustion concept, the fuel/air mixture ignites volumetrically more or less simultaneously in the whole cylinder volume, combustion occurs extremely quickly. Moreover, since no flame propagation is required, unlike what happens in Otto engines, such a HCCI engine can be operated with a high air/fuel ratio λ.
A problem with this state of the art is the actual ignition point, which has to be almost exactly the same as the ignition moment at which the piston reaches its top dead centre. Unlike what happens with Otto or diesel engines (ignition by spark plugs or injection) it is precisely this ignition point that cannot be controlled externally in the case of a HCCI engine. This problem prevents the wide use of HCCI engine technology its advantages consequently remaining unused.